View source for Inotify

{{Short description|Linux subsystem for filesystem monitoring}}
{{Lowercase title}}
'''inotify''' ([[inode]] notify) is a [[Linux kernel]] subsystem created by John McCutchan, which monitors changes to the [[filesystem]], and reports those changes to applications. It can be used to automatically update directory views, reload configuration files, log changes, backup, synchronize, and upload. The ''inotifywait'' and ''inotifywatch'' commands (maintained by Eric Curtin as part of the inotify-tools project) allow using the inotify subsystem from the command line.<ref name="inotify-tools">[https://github.com/inotify-tools/inotify-tools/wiki inotify-tools wiki]</ref> One major use is in [[desktop search]] utilities like [[Beagle (software)|Beagle]], where its functionality permits [[Search engine indexing|reindexing]] of changed files without scanning the filesystem for changes every few minutes, which would be very inefficient.

inotify replaced an earlier facility, [[dnotify]], which had similar goals. Inotify was merged into the [[Linux kernel mainline]] in kernel version 2.6.13, released on August 29, 2005;<ref name="kernelnewbies">[http://kernelnewbies.org/Linux_2_6_13 Linux 2.6.13, kernelnewbies.org]</ref> later kernel versions included further improvements.  The required library interfaces were added into the [[GNU C Library]] (glibc) in its version 2.4, released in March 2006, while the support for inotify was completed in glibc version 2.5, released in September 2006.<ref name="man-page">[http://man7.org/linux/man-pages/man7/inotify.7.html inotify man page]</ref>

== Limitations ==
Limitations imposed by inotify include the following:

* Inotify does not support recursively watching directories, meaning that a separate inotify watch must be created for every subdirectory.<ref>Robert Love, [https://books.google.com/books?id=k_ocKY0iegsC&pg=PA236 Linux system programming]. O'Reilly, p. 236</ref> In contrast, the command ''inotifywait'' does provide recursive watching of directories.<ref>inotifywait man page, [https://man.archlinux.org/man/inotifywait.1#r,]</ref>
* Inotify does report some but not all events in [[sysfs]] and [[procfs]].
* Notification via inotify requires the kernel to be aware of all relevant filesystem events, which is not always possible for networked filesystems such as [[Network File System|NFS]] where changes made by one client are not immediately broadcast to other clients.
* Rename events are not handled directly; i.e., inotify issues two separate events that must be examined and matched in a context of potential race conditions.

== History ==
* July 2004: the first release announcement<ref name="inotify-patch">{{cite web | url = https://groups.google.com/group/fa.linux.kernel/browse_thread/thread/6366aaa10cb23bcc/a54e97d545ad66fe | title = fa.linux.kernel post [RFC][PATCH] inotify 0.8 | date = 2004-07-29 | access-date = 2013-08-19}}</ref>
* August 29, 2005: Linux kernel version 2.6.13 released, containing merged inotify code<ref name="kernelnewbies" />
* March 2006: GNU C Library (glibc) version 2.4 released, bringing initial inotify support<ref name="man-page" />
* September 2006: Glibc version 2.5 released, bringing complete inotify support<ref name="man-page" />

== {{Anchor|FANOTIFY}}Advantages over dnotify ==
There are a number of advantages when using inotify when compared to the older [[dnotify]] API that it replaced.<ref name=lwn-inotify>{{cite web |author=Michael Kerrisk |author-link=Michael Kerrisk |date=14 July 2014 |title=Filesystem notification, part 2: A deeper investigation of inotify |publisher=[[LWN.net]] |url=https://lwn.net/Articles/605128/ }}</ref><ref>{{cite web
  |title=Monitor Linux File System Events with Inotify ; Why inotify?
  |author=Eli M. Dow
  |date=2005-05-31
  |url=http://www.developertutorials.com/tutorials/linux/monitor-linux-inotify-050531/page2.html
  |archive-url=https://web.archive.org/web/20100116105916/http://www.developertutorials.com/tutorials/linux/monitor-linux-inotify-050531/page2.html
  |archive-date=2010-01-16
}}</ref><ref>[https://www.kernel.org/pub/linux/kernel/people/rml/inotify/README inotify README file]</ref> With dnotify, a program had to use one [[file descriptor]] for each directory that it was monitoring. This can become a bottleneck since the limit of file descriptors per process could be reached. Later, fanotify was created to overcome this issue. The use of file descriptors along with dnotify also proved to be a problem when using removable media. Devices could not be unmounted since file descriptors kept the resource busy.

Another drawback of dnotify is the level of granularity, since programmers can only monitor changes at the directory level. To access detailed information about the environmental changes that occur when a notification message is sent, a stat structure must be used; this is considered a necessary evil in that a cache of stat structures has to be maintained, for every new stat structure generated a comparison is run against the cached one.

The inotify API uses fewer file descriptors, allowing programmers to use the established select and poll interface, rather than the signal notification system used by [[dnotify]]. This also makes integration with existing select- or poll-based libraries (like [[GLib]]) easier.

== See also ==
{{Portal|Computer programming|Linux}}
* [[File Alteration Monitor]] (SGI)
* [[DMAPI]]
* [[kqueue]] (FreeBSD, NetBSD, OpenBSD, DragonFly BSD, and macOS)
* [[FSEvents]] (macOS)

==References==
{{Reflist}}

== External links ==
* [http://www.linuxjournal.com/article/8478 Kernel Korner ]{{snd}} an introduction to inotify by Robert Love (2005)
* [https://lwn.net/Articles/104343/ LWN Article on Inotify]{{snd}} watching filesystem events with inotify (partly out of date)
* [http://www.ibm.com/developerworks/linux/library/l-ubuntu-inotify/index.html?ca=drs- IBM Article]{{snd}} monitoring Linux filesystem events with inotify (6 September 2008).
* [https://lwn.net/Articles/604686/ Filesystem notification, part 1: An overview of dnotify and inotify]{{snd}} an LWN.net article by Michael Kerrisk (2014)

{{Linux}}
{{Linux kernel}}

[[Category:Linux kernel features]]